1. Field of the Invention
The present invention relates to a copolyester resin film laminated metal sheet having excellent formability and its method of production. The process comprises laminating a biaxially oriented copolyester resin film, having specified characteristics, precoated with a small amount of a resin composite, the resin composite containing at least one radical, such as an epoxy radical or a hydroxyl radical on one or both sides of a surface treated metal sheet which has been heated to the melting point of the copolyester resin film .+-.50.degree. C. just before the lamination of the copolyester resin film. The side of the copolyester resin film precoated with the resin composite is in contact with the surface treated metal sheet.
2. Brief Description of the Prior Art
At present, metal sheets, such as electrotinplate, tin free steel and aluminum sheets are widely used for can stock after being coated, at least once, with lacquer. This lacquer coating is disadvantageous from an energy standpoint as significant time is required for curing the lacquer and large volumes of solvent discharged during the lacquer curing process must be burned in another furnace in order to prevent air pollution.
Recently, lamination of thermoplastic resin film on a metal sheet was attempted in order to avoid these problems. See for example, Laid-Open Japanese Patent Application No. Sho 53-141786, Japanese Patent Publication No. Sho 60-47103, Laid-Open Japanese Patent Application Nos. Sho 60-168643, Sho 61-20736 and Sho 61-149341.
Laid-Open Japanese Patent Application No. Sho 53-141786 discloses a metal can produced from a metal sheet covered with polyolefin resin film, using an adhesive containing polyolefin resin modified with a carboxyl radical. However, this polyolefin film laminated metal sheet cannot be used as a material for can stock as the metal sheet can become corroded by the packed contents due to the poor permeability resistance of the laminated polyolefin resin film. Furthermore, even if the polyolefin resin film laminated metal sheet is used as a material for can stock, cans having satisfactory appearance cannot be obtained because the laminated polyolefin resin film is melted during heating at temperatures of from 160.degree. to 200.degree. C. These temperatures are required for curing the printing ink or the coated lacquer.
Japanese Patent Publication No. Sho 60-47103 relates to a process for lamination of a crystalline polyester resin film to a metal sheet by heating the sheet above the melting point of said polyester resin film and thereafter immediately quenching the laminate. In this patent, the crystalline polyester film is sufficiently adhered to the metal sheet by an amorphous and non-oriented polyester resin film that is formed at the interface of the crystalline polyester film and the metal sheet as a result of the heating step. However, when the polyester film laminated metal sheet is reheated to 160.degree. to 200.degree. C. for 10 to 30 minutes, as required for curing the printing ink or lacquer applied on the other side of the metal sheet before forming, adhesion of the polyester resin film deteriorates. This is due to the amorphous non-oriented polyester resin layer recrystallizing upon heating. As a result, corrosion resistance also deteriorates.
Laid-Open Japanese Patent Application No. Sho 60-168643 relates to a thermoplastic resin film laminated steel sheet for a drawn and ironed can (DI can) and the production method therefor. In said patent, the side of the steel sheet to be employed for the inside of the DI can is laminated with a thermoplastic resin film, such as polyethylene terephthalate, without any adhesives. The side of the steel sheet to be employed for the outside of the DI can is plated with a ductile metal, such as tin, nickel or aluminum.
The steel sheet according to said patent has the same defects as those in Japanese Patent Publication No. Sho 60-47103, i.e., as a result of the reheating to the 160.degree. to 200.degree. C. for 10 to 30 minutes required for curing the printing ink and the lacquer on the outside of the DI can, the adhesion of the polyester resin film noticeably deteriorates.
Laid-Open Japanese Patent Application Nos. Sho 61-20736 and Sho 61-149341 relate to lamination of a precoated biaxially oriented polyester resin film to a metal sheet heated below the melting point of said polyester resin film. The film is precoated with a special adhesive, such as an epoxy resin containing a curing agent. In said patents, an amorphous and non-oriented polyester resin layer as shown in Japanese Patent Publication No. Sho 60-47103 and Laid-Open Japanese Patent Application No. Sho 60-168643 is not formed. This is because the lamination of biaxially oriented polyester resin film to the metal sheet is carried out below the melting point of said polyester resin film. Therefore, the corrosion resistance and the adhesion of polyester resin film to the metal sheet does not deteriorate, even if it is reheated at temperatures of 160.degree. to 200.degree. C. for the time required for curing printing ink and lacquer. However, if said laminated metal sheet is used for some applications requiring more severe formability, such as a deep drawn can having a drawing ratio higher than 2.0, many cracks occur in the polyester resin film.
Recently, the use of a polyester resin film having specified characteristics has been proposed in order to improve the formability of the polyester resin film laminated metal sheet (Laid-Open Japanese Patent Application No. Sho 64-22530 and Japanese Patent Application No. Sho 63-75837). However, when these polyester resin film laminated metal sheets are formed in processes such as deep drawing or bending at high speed, many cracks arise or the laminated polyester resin film is peeled off in the formed area of said polyester resin film.